(WIP) Sparse models: S-LASSO and TV-l1

- Supports TV-l1 and S-LASSO priors
- Supports logistic and squared losses
- Has cross validation
- Can automatically select alpha by CV (+ automatic computation of useful alpha ranges for the CV)
- Warning: User must supply l1_ratio
Could we have a comment saying `Ideally this loop is broken after one single iteration. It only loops if the precision of the proximal operator is too low, as measured by a non-decrease of the energy functional after the energy-decreasing prox operator. If the maximum of 10 loops is reached, then the dual gap precision is decreased by a factor of (.2) ** 10`